So much unknown. Competition with other survivors with guns, knives, the ability to poison your well when you aren’t looking or burn your house down while you sleep…

Or, everyone rallies in the first few years and the few survivors that are around work together and share resources, at least long enough that everyone gets theirs.

Movies and video games are more entertaining when there's over-the-top conflict, and I think that shapes our predictions. We have seen, however, that in times of crisis communities and people rally together for the greater good. Even strangers help strangers. That doesn't usually last long, but it does happen. And I have no reason to believe that wouldn't be true if a world-killing meteor struck the earth in China. On my side of the planet, I do believe there would be a honeymoon phase before mad Max times.

I'd be naive to assume that it would be without conflict for generations. I'm not saying that at all.

Even the happy cooperative commune is going to need centuries of relative peace in order to reboot the supply chain to the point of making new spark plugs compatible with the old engine blocks - or new engine blocks when the old ones are too worn to rebuild.

Yes. It would take a while to figure out how to reboot factories and supply chains (if that even is the goal after the extinction event -- when there's 10,000 left alive scattered throughout the world, why would we need factories and supply chains?). But also, there are more than enough spark plugs and engine blocks packed in oil at tractor supplies and John Deere service departments to make it happen for generations. Traveling may or may not be dangerous -- neither you nor I know.

My other claim is that we'd learn to use horses/cows to pull the tractor implements before the tractors are kaput for good. I stand by that claim.

The biggest danger would be illness and injury.